Compressed air supply installations are used in all kinds of vehicles, particularly to supply vehicle air suspension systems with compressed air. Air suspension systems may also include level control mechanisms, with which the distance between the vehicle axle and the vehicle body can be set. An air suspension system in a pneumatic system comprises a number of air bellows pneumatically connected to a common line (header), which lift the vehicle body as they fill up and lower it as they deflate. As the distance between the vehicle axle and the vehicle body, or ground clearance, increases, the spring deflections become longer and even relatively significant surface unevenness can be overcome without coming into contact with the vehicle body. Systems of this kind are used in off-road vehicles and sports utility vehicles (SUVs). Particularly in the case of SUVs with very powerful engines, it is desirable for the vehicle to be provided, on the one hand, with comparatively low ground clearance for high-speed road driving and, on the other hand, with comparatively high ground clearance for off-road driving. It is furthermore desirable for a change in ground clearance to be made as quickly as possible, thereby increasing demands in relation to the speed, flexibility and reliability of a compressed air supply installation.
A compressed air supply installation for use in a pneumatic system with a pneumatic unit, for example an air suspension system, is operated using compressed air from a compressed air supply, within the limits of a pressure level of 5 to 20 bar, for example. The compressed air is made available to the compressed air supply using an air compressor. The compressed air supply is pneumatically connected to a compressed air connection to supply the pneumatic unit and, on the other hand, is pneumatically connected to a vent port. By means of a vent valve arrangement, the compressed air supply installation and/or pneumatic unit can be vented towards the vent port by releasing air.
In order to realize long-term operation of the compressed air supply installation, the system comprises an air drier, with which the compressed air can be dried. The accumulation of moisture in the pneumatic system, which can otherwise lead to valve-damaging crystal formation and other undesirable effects in the compressed air supply installation and in the pneumatic unit at comparatively low temperatures, is thereby avoided. An air drier comprises a drying agent, customarily a granulate fill, through which the compressed air can flow, so that the granulate fill can adsorb moisture contained in the compressed air at comparatively high pressure. An air drier may, where appropriate, be designed as a regenerative air drier. This involves dried compressed air from the air suspension system being passed through the granulate fill at comparatively low pressure in counter-current or co-current flow relative to the filling direction during each venting cycle. The vent valve arrangement may be opened for this purpose. For an application of this kind—also referred to as pressure swing adsorption—it is desirable for a compressed air supply installation to be flexibly and, at the same time, reliably designed. In particular, comparatively rapid venting should be made possible, yet there should be a sufficient pressure swing to allow regeneration of the air drier.
A solenoid valve for implementing a multiple function may be realized with at least three connected pneumatic chambers—namely, for the pneumatic connection of functionally different pneumatic chambers, which are always separate and have different pressures applied to them—having two mechanically separate lifting armatures in a common exciter winding of the solenoid valve. Each of the lifting armatures is allocated to a different separate pneumatic chamber in this case. A dual-armature solenoid valve of this kind is disclosed in principle in DE 201 60 30, for example, or in the present applicant's DE 35 01 708 A1 or DE 10 2006 041 010 A1, for example. The basic method of operation of a dual-armature solenoid valve can be derived from the applicant's DE 10 2004 035 763 A1, for example.
The applicant's DE 35 429 74 A1 discloses a level control mechanism for vehicles with air filters, with which a predetermined distance between the vehicle cell and the vehicle axle can be maintained by inflating or deflating the air springs according to the vehicle load. The mechanism contains a safety valve that can be controlled using the pressure in the air springs. A compressed air supply installation of this kind can be further improved.
DE 199 11 933 B4 discloses a compressed air generator having an air drier with a first compressed air supply line, wherein the compressed air is conducted through a drying agent, and with a second compressed air supply line, in which the compressed air is not conducted through the drying agent.
A compressed air supply installation of the general type under consideration is also disclosed in EP 1 165 333 B2 in the context of a pneumatic system with an air suspension system. Apart from a separately closable main venting line, this exhibits a high-pressure venting line with an additional high-pressure vent valve alongside the main vent valve pneumatically actuated using a control valve in the main venting line and which is connected in parallel to the main venting line. The free flow-through cross-section of the separate high-pressure vent valve is smaller than that of the main vent valve. A compressed air supply installation of this kind can also be further improved. When a compressed air supply installation of this kind is vented via the high-pressure venting line, dry air is vented, which is not used to regenerate the drying agent. This represents an unnecessary waste of dry air, particularly in the event that a suitable flexible, rapid and yet reliable actuation of the compressed air supply installation with a correspondingly high actuation rate should be necessary for the above applications. Although a main vent valve that can be configured with a comparatively large nominal width can be pneumatically pre-controlled via the control valve provided with a comparatively small nominal width, an arrangement of this kind designed for the indirect connection of a pressure volume is comparatively expensive.
A compressed air control mechanism with an air drier is disclosed in EP 1 233 183 B1. The housing of the air drier includes a cup-shaped drying container, the inside of which is connectable via the housing firstly to a pressurizing agent source and secondly to a connection element in the form of a pressure reservoir and/or an air spring via a valve opening. The housing exhibits an air inlet and an air outlet for compressed air in each case, the air being conducted in a flow direction from the air inlet through the drying container to the air outlet, in order to inflate the connection element. For deflation, the compressed air is removed in the opposite flow direction from the air outlet through the drying container and the housing. A controllable directional valve is integrated and fitted into the housing of the air drier, the valve serving to allow the air into the inside of the housing and the drying container during deflation. A directional valve controlling the discharge channel is actuated by at least one further controllable directional valve using the pressure during deflation. This directional valve is arranged substantially outside the housing of the air drier.
A compressed air system with an outlet valve that can be charged by a compressor and exhibits a compressed air unit is disclosed in DE 32 16 329 C2, wherein a pressure-retention valve is provided in a flow-back line between the compressed air storage container, on the one hand, and the container holding a drying agent and also the outlet valve, on the other. The outlet valve and a pressure regulator regulating it are continuously connected to a hollow rod by the container holding the drying agent.
These and other solutions from the prior art for an air drier have proved to be comparatively demanding in terms of installation space. What is needed is a compressed air supply unit having a vent valve arrangement and an air drier that can be set up in the most space-saving manner possible.